The reduction of titanium dioxide (TiO2) by the hydrogen plasma treatment creates a new absorption band in the visible light region and is expected to create photocatalytic activity in this region. Anatase TiO2 powder, which has photocatalytic activity in the ultraviolet (LTV) region, was treated using microwave and radio-frequency (RF) plasmas. The TiO2 was reduced thus having an oxygen deficiency. The RF-plasma-treated TiO2 absorbed visible light between 400 and 600 nm and showed a high activity in this region, as measured by the oxidative removal of nitric oxide from the gas phase. The plasma-treated TiO2, which can be easily produced on a large scale, is expected to have a higher efficiency in utilizing solar energy than the raw material.